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Phymaturus: Lizards running on ash

After many months, the last of the tropidurines has arrived. Phymaturus is a very particular genus of lizards and stands out of the rest of the genera in Tropidurinae because of its particular morphology, adapted to a life in rocky micro-habitats. In general, the lizards from this genus will show very flat bodies and heads, as if they had been smashed from above. This can make their appearance to be strange as well as fascinating to us.

Making some comparisons with other genera, the results of a Principal Component Analysis (after removing the effect of body size) can help to illustrate how is that Phymaturus differences from the rest of the clade.

Principal component 1 (PC1) happens to be negatively related in this case with all the characteristics related with limb length. This means that genera which, in average, show long limbs should obtain negative values for this PC. Besides showing a flat body as I wrote before, Phymaturus lizards have quite short limbs which is why the show positive values for this PC. Actually, they show the highest value for the whole clade.

PC2 happened to be related, among other characteristics, to the height of the body and the head. This means that genera with relatively high heads and bodies will show positive values in this PC. Phymaturus, as we see, obtains negative values, which fits our first impression about this lizards.

However, this time I’m going to write about another characteristic of at least some species of the genus, and it is that these lizards are constantly subjected to the consequences of volcanic eruptions. A paper of 2016 (Ibargüengoytía et al., 2016) describes how this kind of event can affect these lizards.

In 2011 the volcanic complex Pueyhue-Cordón Caulle erupted throwing tens of millions of tons of ash to the air. This ash then accumulated up to 30cm in the surrounding grounds. These surroundings are the habitat of the species of this study. Because the ash totally changes the substrate in which these animals move, some experiments were done to determine how the ash deposition affected the speed of the Phymaturus lizards when compared to normal substrate. Besides the effect of the ash, the effect of the slope (inclination of the substrate) and of the interaction between both variables was measured. Why is it important if a lizard can move faster or slower? The answer is that the locomotion speed is related with fitness because it will affect the ability of an individual to catch prey, flee from predators, maintain a territory, etc. If the ash reduces the speed of these lizards, then we could say that is a fact that it is negatively affecting them.

In addition to the effects in the locomotion some comparisons were made in respect to the body condition of the lizards before and after the volcanic eruption, being the mass of the individuals used as an indicative of this variable. The same population was sampled before (2010) and after the eruption (2012) to determine if there was any effect.

In summary, the study describes (a) the change in body condition produced by the eruption, and how (b) the ash, (c) the inclination and (d) the interaction between both affect locomotion.

However, there is another important point to take into account. The study considered two species, which present distinct adaptations to particular micro-habitats. Phymaturus sinervoi often inhabits zones with flat rocks surrounded by sand, while P. excelsus is adapted almost exclusively to rocky habitats. This information will be relevant when looking at the results.

Finally, the examined populations came from zones in which the substrate was affected in a different way by the ash deposition. In the next figure we see that the evaluated population of P. sinervoi (square) was found in a zone where the ash covered the ground to a lesser extend in comparison to the zone where P. excelsus was sampled (triangle).

Sampling localities for the species of the study (square = P. sinervoi, triangle = P. excelsus, circle = P. spectabilis, not part of this study) and level of ash deposition. Extracted from Ibargüengoytía et al., 2016

With this in mind, the results showed the following.

We could expect that the volcanic eruption and its consequences would affect the generations appearing in the following months. However, the results showed the opposite. For P. excelsus the body condition was better in the individuals that were evaluated after the eruption than in those living before it. This result was the same for males, females and juveniles. In P. sinervoi there was no significant change in the body condition after the eruption in neither males, females or juveniles.

What could have happened?

it is know that the lack of rain in Patagonia makes difficult the rapid assimilation of ash in the ground. This makes the richness and abundance of vegetation to diminish, affecting the lizards that feed from it and use it as refuge. A previous study, which also evaluated the effect of volcanic eruptions in another Phymaturus species, reported a poor body condition of the lizards of this species, in addition to a low number of reproductive females (Boretto et al., 2014). All of this a year after a volcanic eruption event.

The authors explain these new results in an interesting way. Lizards have to compete with livestock, often sheep and goats, for plant material. However, ash also affects livestock, and even harder when compared to lizards. This because, apparently, ash would have highly detrimental effects in the digestive system of these animals, increasing their mortality. Consequently, more resources would be available for Phymaturus lizards.

Why does P. sinervoi did not show an improvement in its body condition like P. excelsus? If we go back to the previous map we see that the amount of ash that was deposited in the zone where P. sinervoi was sampled was smaller than that deposited in the sampling zone of P. excelsus. This smaller amount might have meant that goats and sheep also showed a lower mortality, which allowed them to maintain their influence on that zone’s resources.

Interesting to see how an apparently negative event for a species can bring it benefits through detrimental effects over its competitors!

Having explained the patterns of body condition now we have to explain the patterns of locomotion, and here is where we have to remember the particular adaptations of each species.

Logically, one would expect ash to negatively affect the speed of these lizards because it reduces the friction between the body surface and the substrate. This is exactly what was found for P. excelsus. Speed, either in short or long distances (the study included both variables, but we can consider them just as locomotion capacity), was mainly affected by ash, either in inclined or on-level surfaces (look triangles and dashed lines in the next figure). Notice how even the inclination seems not to have any effect when we compare the speed in short distances for this species.

The effect of different treatments on the speed of sprint runs (A) and long runs (B). Extracted from Ibargüengoytía et al., 2016.

In conclusion: In Phymaturus excelsus speed is affected by ash deposition, but not by inclination.

However, a different pattern is observed for P. sinervoi (squares and solid lines in the figure). For this species the effect of ash is small or inexistent in comparison with the effect of inclination, which makes these lizards quite slow.

In conclusion: In Phymaturus sinervoi speed is affected by inclination, but not by ash deposition.

Maybe the explanation of these patterns is related to the adaptations of these two species to particular micro-habitats. As I wrote before, P. sinervoi is a species adapted to micro-habitats with flat rocks surrounded by sand, so we could expect that individuals of this species are adapted to move in solid and loose substrates, as long as they are flat.

In contrast, P. excelsus is a species adapted to rocky substrates, with surfaces of different slopes. This could explain why the inclination does not seem to be a problem for speed, unlike ash, which makes the substrate to be loose, opposite to what this species is used to.

All of these results show quite complicated interactions between micro-habitat adaptations, resources offer, interspecific competition, functional ecology and finally circumstantial events, like volcanic eruptions. But, in spite of these hostile conditions, Phymaturus lizards seem to be well adapted to the environmental pressures they face either each day or each 50 years.

When I thought about what would I write about Phymaturus I imagined that I would probably find something related to their flat body and how much does it helps them to live between rocks, or about their herbivory (which is quite rare among reptiles), or about the fact that they’re viviparous. However their relationship with the typical volcanic activity of the zones their inhabit was equally interesting. With all of these bizarre characteristics, could we say that Phymaturus is the most strange genus of the clade?

In any case, I think it was a nice end to this series. Now that I wrote about each of the 10 genera I studied is time to determine who is who within the clade Tropidurinae in terms of morphology, and in general how this morphological diversity contrasts with such an important ecological characteristic as habitat use.